This invention relates to a method for making porous tantalum pellets, and more particularly to the production of such pellets for use in electrolytic capacitors.
The usual method for making porous tantalum pellets includes coating tantalum powder particles with an organic binder medium, dispensing the coated powder into a mold-press cavity, compressing the powder to produce a pellet, heating the pellet under vacuum to remove the organic binder, and vacuum sintering the pellet.
The tantalum powder, as initially received, has a thin tantalum oxide film over the surfaces of all the particles which film tends to inhibit cohesion between the particles upon pelleting, thus making it necessary to use a binder. However the organic binder coating after heat sintering leaves a residue of carbon on the particle surfaces even after the above noted heating and sintering steps, which carbon residue tends to increase the leakage currents of an electrolytic capacitor made from such conventional tantalum pellets.
Although it is known to form compacts of tantalum powder without a binder, the green strength of such unsupported compacts or pellets is generally weak and difficult to handle without breakage during subsequent manufacturing steps.
It is therefore an object of this invention to overcome the above mentioned shortcomings of the prior art.
It is a further object of this invention to provide a porous tantalum pellet having essentially no carbon residue on the tantalum surfaces thereof.
It is a further object of this invention to provide a porous tantalum pellet having a greater tantalum surface area per unit weight of tantalum than heretofore.